What is forest interpretation of aerial photos?

(forest interpretation of aerial photos)

(Ma Jianwei)

The operation of forest investigation using aerial photos. That is, according to the imaging law of forest features, qualitative and quantitative information of forest land types and forest investigation factors are extracted from aerial photographs by analyzing the image characteristics of aerial photographs and using forestry professional knowledge. It can be divided into: ① contour interpretation: according to the requirements of forestry management, the contour lines of various land types and forest subcompartments in forest areas are drawn on aerial photographs for investigation, statistics or management. This is a qualitative identification of land types. ② Tree-measuring interpretation: On the basis of contour interpretation, the tree-measuring factors such as forest composition, forest age, canopy density, tree height, volume and so on are interpreted or measured. It is a quantitative evaluation of tree measuring factors in the stand. The purpose of interpretation is to obtain qualitative and quantitative data of various investigation factors in the small class on the photos, and to provide basis for forest investigation.

Basic elements of forest interpretation

Aerial photography images are formed by electromagnetic radiation reflected and emitted by ground objects, which can make films sensitive. Different objects have different physical properties, and their spectral characteristics of reflection and emission are also different. Forest remote sensing imaging mainly uses this characteristic, and projects the reflected light of different land types, tree species and different parts of the forest onto the photosensitive film through the camera objective lens to form different images. Therefore, aerial photographs record the shape, size, color (hue), shadow and the relationship between ground objects, reflect the actual situation of ground objects, and are the basic factors for interpretation, which can be used to identify ground objects.

Ground object shape

Ground objects and their images have specific shapes. Their changing laws change according to the central projection law. On the aerial photograph of vertical photography, the shape of the ground objects in the flat area is similar to the image. The vertical ground object image is deformed due to poor projection, and its image shape changes according to the following rules: ① On the vertical aerial photograph, the three-dimensional image of the ground object near the central part of the photograph stands upright, showing the top shape of the object; The stereoscopic image of the edge part of the photo is inclined, showing the side shape of the object. ② The farther an object or tree with the same height is from the center of the image, the longer the image, that is, the greater the projection difference. (3) The images of ground objects or trees are inclined radially outward and extend through their vertices and bottoms, and then all meet at the bottom of the images. Forest interpretation applies these laws to explain forest investigation factors. For example, when reading the canopy density, the reading result of the stand with the same canopy density will be higher at the edge of a single photo, while the reading result of the photo center in a stereoscopic image will be more accurate.

Physical size

Refers to the relationship between the size of unknown features and the size of known features. The actual length of an unknown object can be determined by the length of a known object in a photo.

Looking for harmonious colors

The hue of black and white film is the degree to which the reflected light of ground objects is projected on the silver salt emulsion layer, and it turns black through photochemical reaction. The tone on the negative is density difference; On the positive film, it is the brightness difference of the object image. According to the spectral reflectivity of ground objects, human eyes can distinguish 10 tone levels. The depth of hue depends on the reflection ability of ground objects. Features with strong reflection ability are light in color on the positive film; On the contrary, the tone is dark. Color is the stimulating feeling of light with different wavelengths in the visible spectrum to the color sensing unit of human eyes. For example, in sunlight, green leaves reflect light with a wavelength of 480-560 nanometers, while relatively absorbing light with other wavelengths, so the object appears green. It is red when it absorbs other light and reflects red light. The color of an object depends on the spectral composition of the light source and the proportion of light with various wavelengths reflected and absorbed by the surface of the object. When an object reflects more light in a certain band than other bands, the object will display the color of the band with the highest reflectivity. Readers' ability to distinguish colors in color aerial photos is about 100 times greater than that in black and white photos. Tone and color change according to factors such as photography season, lighting conditions, film type and photography processing. Comparison methods such as optical wedge or tone module are usually used to determine the tone of an image. Photoelectric densitometers can also be used to determine the density of photos and determine different features or positions.

Ground shadow

That is the shadow itself and the shadow cast. The former is formed by the backlight surface of the object itself. In vertical photography, it is the image of the backlight side of the object, which is obviously different from the color tone of the image of the light receiving side. Characterized by the transition from the light receiving surface to the backlight surface and the proportional relationship between them. This feature can be used to explain the crown shape. The image of circular or flat crown gradually transits from the light receiving surface to the backlight surface; The transition of pointed cone crown is obvious. A cast shadow is a shadow cast on the ground. Its characteristic is that it can display the longitudinal shape of objects and cast shadows on flat areas without deformation. This feature can be used to explain the shape and size of crown and distinguish tree species from ground objects. The height of the tree can be determined according to the length of the shadow. The shadow length varies with the solar altitude angle and terrain fluctuation. Trees covered with shadows reflect very little light, which makes the image in the photo very dark or even not reflected.

Shadow structure and pattern

The schlieren structure of a photo refers to the tone or the frequency of tone change. It is the result of the hue or hue change of those small objects, which are too small to be clearly distinguished by themselves. In large-scale photos, individual trees can be distinguished, but their leaves can't be distinguished. The shadow structure of the crown is composed of the reflection of the leaves. Some subtle objects can't be shown in photos. Such as river cutting erosion, can only be judged by the shadow structure in small-scale photos. Pattern refers to the special arrangement of objects, which is formed naturally and artificially. The outcrop shape of rocks on the ground often indicates geological structure; The type of water system is closely related to structure, lithology and soil structure. The types of forest plants are closely related to their community types. According to these characteristics, we can distinguish them properly in photos.

other

Because the crown shapes, sizes, colors and distribution positions of different tree species are different, different camera structures are formed in the photos. It is mainly manifested in the shape, size, color tone of crown image and the height difference of forest image. According to these differences, we can interpret the composition of tree species and trees in photos. In addition, the distribution of tree species and stands has certain regularity. By mastering these laws and combining with other interpretation factors, we can correctly identify tree species and stand composition.

The conventional forest visual interpretation method is based on the above-mentioned interpretation factors such as shape, size, color and shadow, and adopts three-dimensional observation method. Readers can get a high sense and see a three-dimensional model. Very conducive to interpretation. Before interpretation, you should be familiar with the situation of the interpretation area: ① topographic characteristics, stand status, tree species and their distribution; (2) Spectral characteristics, spatial characteristics, time-varying characteristics, types and processing of aerial photographic films of stands and ground objects. And collect data and tools related to interpretation, such as topographic map, forest map, interpretation module, interpretation table, etc. Interpretation is generally carried out in the following order: ① Water system interpretation, to determine its location and flow direction; (2) Distinguish watersheds, determine the location of watersheds according to the water system, and master the topographic features; (3) Interpret easily identifiable land types such as farmland first, and then interpret difficult land types; (4) According to the distribution of water system, taking residential areas as control, determine the position of main traffic lines between residential areas; ⑤ Explain subtle features, such as various investigation factors of small classes in woodland.

Forest interpretation content

① Interpretation of land types: including rivers, farmland, logging areas, etc. The image of the river shows many curved areas; Panchromatic photos vary from gray to light black, depending on the lighting conditions, water surface conditions and river depth during photography. The image of the road is thin and its color changes with the smoothness of the road surface. The railway is white or light gray linear, and the curvature radius of the bend is large. The images of residential areas are composed of images of buildings with different shapes and sizes, which are generally arranged in a certain order. The images of farmland are mostly geometric figures, ranging from white to dark gray or black, depending on soil type, soil moisture, growing crops and photography season. The image of the swamp is irregular in shape and mostly distributed in the river valley, showing a gray-black picture. The grassland is irregular in shape, even gray or dark gray, and it is located near rivers or low humidity valleys. The image of the burned land is irregular, with cracked edges, extending to the woodland, showing light gray or light gray. The image modeling of clear-cut land is a regular geometric figure with a light tone and a cluster of mother trees. The image is light gray and the shadow cast is light black. On large-scale photos, you can see slippery roads and piled branches. The image of trees in sparse forest land is sparse, which is composed of gray forest land and shadows. The shadows cast are complete and obvious, and the situation of reforestation can be seen in large and medium-scale photos. ② Interpretation of forest land: including interpretation of tree species composition, age group, crown width, tree height and forest density in aerial photos.